An ethylene/tetrafluoroethylene copolymer (hereinafter sometimes referred to as ETFE) is excellent in heat resistance, chemical resistance, electrical insulating properties, flame retardancy, weather resistance and molding processability and is used as an insulating coating material for electric wires to be used for aircraft, atomic power plants, automobiles or industrial robots.
When ETFE obtained by a polymerization method such as solution polymerization or suspension polymerization is granulated, the handling efficiency of ETFE obtained by the polymerization will be improved, and clogging by ETFE in the carrier piping to an extruder or during pipe transportation is less likely to occur at the time of pelletizing ETFE by melt extrusion. Further, also at the time of processing ETFE into a fine powder having a desired particle size by pulverizing it, granulated ETFE is excellent in handling efficiency, such being desirable.
As a method for granulating ETFE, a method is known wherein after copolymerizing ethylene, tetrafluoroethylene and, if necessary, other comonomers, by a polymerization method such as solution polymerization or suspension polymerization, unreacted monomer gases are purged, and then a slurry of ETFE is granulated to obtain ETFE granules (Patent Document 1).
In order to produce ETFE granules efficiently and at low cost, it is preferred to transport the ETFE slurry from the polymerization tank to the granulation tank by the monomer gas pressure, since an equipment such as a transfer pump may be omitted.
However, if, in the granulation tank, ETFE is granulated in the presence of monomer gases of ethylene and tetrafluoroethylene, while recovering such monomer gases and the polymerization medium, an ETFE oligomer is likely to be formed in the granulation tank, and there has been a case where the properties of ETFE are not adequate depending upon the particular application of ETFE. Further, there has been a case where a piping is clogged, since fine particles of ETFE are contained in the ETFE granules depending upon the granulation conditions.